With the remarkable proliferation of portable appliances such as personal computers and cellular phones, there is an increasing demand for batteries used as the power source for portable appliances. Among the batteries used in such applications, lithium ion secondary batteries have high energy density and excellent cycle characteristics, so demand for lithium ion secondary batteries is particularly increasing. Lithium ion secondary batteries usually include a positive electrode including a lithium-containing composite oxide, a negative electrode including lithium metal, a lithium alloy, or a negative electrode active material that absorbs and releases lithium ions, and a non-aqueous electrolyte. Recently, in an attempt to further improve the performance of lithium ion secondary batteries, the use of elements capable of absorbing lithium ions (hereinafter referred to as “Li-absorbing elements”) as negative electrode active materials has been studied.
Li-absorbing elements absorb lithium, for example, by alloying with lithium. Among them, silicon and tin, which have high theoretical capacity densities of reversible absorption and release of lithium (hereinafter referred to as simply “theoretical capacity density”), are viewed as promising negative electrode active materials. Currently, various lithium ion secondary batteries using Li-absorbing elements as negative electrode active materials are proposed. For example, there has been proposed a lithium ion secondary battery having a negative electrode that includes a negative electrode current collector and a negative electrode active material layer made of a silicon amorphous or microcrystalline thin film formed on the surface of the negative electrode current collector (e.g., see Patent Document 1 and Patent Document 2). In these Patent Documents, the silicon thin film is formed, for example, by chemical vapor deposition (CVD) or evaporation.
However, when Li-absorbing elements absorb and release lithium ions, they repeatedly expand and contract and undergo large volume changes, and this property of Li-absorbing elements limits their use as negative electrode active materials. That is, when a Li-absorbing element is included in a negative electrode active material layer of a negative electrode, the negative electrode active material layer undergoes large volume changes due to the absorption and release of lithium ions, thereby creating a large stress. Such a negative electrode is subject to deformation such as distortion, wrinkles, or breakage. Also, it is highly likely that the negative electrode active material layer becomes separated from the negative electrode current collector. This may result in creation of space between the negative electrode and the separator or the negative electrode active material layer, uneven charge/discharge reaction, and degradation of cycle characteristics.
To solve these problems associated with Li-absorbing elements, it has been proposed to form a space in a negative electrode active material layer for easing the stress created by the expansion of a Li-absorbing element, in order to suppress the deformation of the negative electrode or the separation of the negative electrode active material layer (see, for example, Patent Documents 3 to 5). Patent Documents 3 to 5 propose a negative electrode including a negative electrode current collector and a negative electrode active material layer made of a plurality of columns comprising a Li-absorbing element which are joined together. The plurality of columns are formed in a certain pattern on the surface of the negative electrode active material layer in such a manner that they extend in the direction perpendicular to said surface and are spaced apart from one another. In Patent Document 3, columns are formed by photoresist method. In Patent Document 4, columns are formed by etching. In Patent Document 5, in forming columns, mesh is placed between a deposition source of a Li-absorbing element and a negative electrode current collector to provide the surface of the negative electrode current collector with an area onto which the Li-absorbing element deposits and an area onto which it does not deposit. When a negative electrode as described in Patent Documents 3 to 4 is mounted in a lithium ion secondary battery, a major part of the surface of the positive electrode active material layer faces the exposed part of the negative electrode current collector rather than facing the negative electrode active material layer. Thus, the lithium supplied from the positive electrode active material layer during charging tends to deposit on the exposed part of the negative electrode current collector without being absorbed into the negative electrode active material layer. As a result, the lithium is not efficiently released from the negative electrode during discharging, the coulombic efficiency and the capacity retention rate lower, and eventually the cycle characteristics degrade. Further, the battery safety also lowers.
Also, there has been proposed a negative electrode that is composed of a thin film of a Li-absorbing element formed on the surface of a negative electrode current collector and columns of the Li-absorbing element formed on the thin film, and the area of these columns in contact with the thin film is 2×10−7 m2 or less (e.g., see Patent Document 6). In this negative electrode, deposition of lithium on the negative electrode current collector surface during charging is prevented. However, since the area of the negative electrode current collector surface other than the area where the columns are formed is also covered with the thin film of the Li-absorbing element, expansion of the Li-absorbing element cannot be sufficiently eased. Hence, deformation of the negative electrode may occur.
Further, there has been proposed a negative electrode including a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector, and this negative electrode active material layer is composed of a plurality of columns that are spaced apart from one another and include a Li-absorbing element, and the columns are grown slantwise from the surface of the negative electrode current collector at an angle relative to the direction perpendicular to said surface (see, for example, Patent Document 7). In Patent Document 7, since the plurality of columns are formed slantwise, the surface of the negative electrode current collector does not directly face the positive electrode active material layer, and the lithium released from the positive electrode active material layer is efficiently absorbed into the negative electrode active material layer. Also, since the columns are spaced apart from one another, the stress due to the expansion of the Li-absorbing element can be eased, so that the deformation of the negative electrode is sufficiently prevented.
However, the technique of Patent Document 7 needs to be improved in terms of preventing separation of the columns from the negative electrode current collector. That is, when a plurality of columns are formed in a certain pattern on the surface of a negative electrode current collector, the contact area of the negative electrode current collector and the columns will become small. Thus, the bonding strength between the negative electrode current collector and the columns becomes low, so that the columns become separated from the negative electrode current collector, which may result in degradation of charge/discharge characteristics. Such a technical problem also exists in the negative electrodes of Patent Documents 3 to 6. Further, Patent Document 7 also describes a technique of forming depressions and protrusions on the surface of a negative electrode current collector and growing columns from the surfaces of the protrusions. In this case, the columns are also susceptible to separation in the same manner as described above. Further, in this technique, columns may also grow from the surfaces of the depressions. If columns grow from the surfaces of the depressions, the gaps between the columns become small, so that sufficient pores cannot be formed in the negative electrode active material layer. Hence, the stress due to the expansion of the Li-absorbing element may not be sufficiently reduced.
Further, according to the technique of Patent Document 7, since the columns are always inclined in a certain direction, the interval between the positive electrode active material layer and the columns becomes relatively long, compared with the negative electrodes of Patent Documents 3 to 6. This means that the lithium ions have to move a long distance. Thus, particularly in discharge, large-current discharge cannot be sufficiently carried out, and the discharge capacity at low temperatures may become insufficient.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-83594    Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-313319    Patent Document 3: Japanese Laid-Open Patent Publication No. 2004-127561    Patent Document 4: Japanese Laid-Open Patent Publication No. 2003-17040    Patent Document 5: Japanese Laid-Open Patent Publication No. 2002-279974    Patent Document 6: Japanese Laid-Open Patent Publication No. 2003-303586    Patent Document 7: Japanese Laid-Open Patent Publication No. 2005-196970